1. Field of the Invention
The present invention relates to a scanning optical device and an image forming device using the scanning optical apparatus. More particularly, the present invention relates to a scanning optical device suitable to an apparatus, such as a laser beam printer or a digital copying machine, in which image formation is conducted using a laser beam produced from light emitting points of a plurality of light sources in order to achieve high speed operation and an increase in recording density, in particular, which uses an over field scanning optical system (hereinafter referred to as an OFS scanning optical system) for receiving a light beam emitted from a light source unit on a wider region than a deflection surface in a main scanning direction and scanning a surface of a photosensitive member using light.
2. Related Background Art
In recent years, high speed operation has been demanded in a digital copying machine and a laser beam printer. In order to meet the demand, such inventions have been made, which include an OFS optical system capable of scanning using the increased number of deflection surfaces of a polygon mirror as a deflector and a multi-beam scanning optical device that can simultaneously scan a plurality of scanning lines by using the increased number of light sources.
In general, an element used for a light source is a semiconductor laser and a light beam having a Gaussian distribution is emitted from the light emitting point so as to have a given spreading width.
After the emitted light beam is formed into a light beam at a spreading angle, which is regulated by an incident optical system, a part of the incident light beam is cut away by the deflection surfaces of the polygon mirror to form a scanning light beam. Because the light beam is fixed against the rotated deflection surfaces, a usage region of the light beam to be used is changed according to the rotation.
When a cutaway manner of the light beam having the Gaussian distribution is momently changed, a distribution of intensity of the scanning light beam is produced. The distribution becomes an illumination distribution on a surface to be scanned, thereby causing unevenness in density of an image.
In order to solve the above problem, various correction techniques have been created, such as a technique using a gradation ND filter disclosed in Japanese Patent Application Laid-Open No. 11-014923 and a technique for conducting electrical correction disclosed in Japanese Patent Application Laid-Open No. 04-255874. Further, an idea is devised in Japanese Patent Application Laid-Open No. 11-218702, in which an opening portion is provided in an incident side to simplify a structure.
According to the above-mentioned techniques, the illumination distribution can be improved. However, there is a demerit in that structures of parts are complicated and a cost is increased.
In the technique in which the ND filter or the opening portion is provided, there is also a problem in that a sufficient effect cannot be obtained depending on processing precision of parts, an assembly tolerance, or a variation in parts.
However, in the above-mentioned various techniques, it is assumed that the light source is disposed in a normal state. When a chip of the semiconductor laser as the light source is assembled so as to be tilted toward a main scanning direction with respect to an optical axis, the center of intensity of a substantially parallel light beam, which has emerged from a collimator lens, is shifted from the optical axis. Thus, an intensity distribution of the incident light beam, which is cut away by the polygon mirror, becomes asymmetrical.
In an extreme case, an intensity peak of a scanning line formed on a surface to be scanned is located outside a scanning effective region and the intensity distribution is tilted, so that the intensity at the peak becomes two or more times larger than an initial intensity.
When an OFS scanning optical device using a plurality of light sources is considered, it is expected that distributions of light emitted from respective semiconductor lasers are different from one another. Thus, even if all light beams are aligned by the polygon mirror deflection surfaces as deflection surfaces, when scanning is conducted using such a light beam, an illumination distribution is changed for each scanning line. Therefore, there occurs a situation where the entire image has an uneven density.
It is difficult to prevent the above problem by measures according to the above-mentioned techniques.
The present invention has been made in view of the above-mentioned problems. Therefore, an object of the present invention is to provide a scanning optical device in which distributions and exit axes of light beams emitted from light sources are specified when scanning is conducted by an OFS scanning optical device using a plurality of light sources, so that an illumination distribution on a surface to be scanned is made substantially symmetric, thereby obtaining a high image quality and high speed operation simultaneously.
Also, another object of the present invention is to provide a scanning optical device capable of making the illumination distribution substantially symmetric with respect to a scanning center axis while the extension of a structure according to the improvement of an image quality is minimized to suppress an increase in cost.
In order to attain the above-mentioned objects, according to a first aspect of the present invention, there is provided a scanning optical device including: a first optical system for guiding at least two light beams emitted from at least two light emitting points to deflection means; and a second optical system for imaging as light spots the plurality of light beams deflected-by the deflection means on a surface to be scanned, characterized in that:
the light beams incident to a deflection surface of the deflection means are set to light beams wider than the deflection surface in a main scanning direction; and
in a main scanning section, when respective angles formed between the center axes of emitted distributions of the light beams from the at least two light emitting points and a corresponding optical axis of the first optical system are given by xcex1i, the angles among xcex1i with respect to two light beams located at the longest distance from the optical axis are each given by xcex11 and xcex12, a maximum difference formed between the angles of xcex1i is given by xcex1xcex5, respective total angular widths of radiation angles of the light beams from the at least two light emitting points within the main scanning section are given by xcex8i, two beam radiation angles in which a difference of the total angular widths is maximum are given by xcex81 and xcex82, and a difference of the beam radiation angles is given by xcex8xcex5=|xcex81xe2x88x92xcex82|,
2xe2x89xa7|xcex11xe2x88x92xcex12|xe2x89xa70 
4xe2x89xa7|xcex11+xcex12|xe2x89xa70, and 
2xe2x89xa7xcex1xcex5/(1+xcex8xcex5)xe2x89xa70 
are satisfied, where xcex8i is a range up to 1/e2 of a peak of an intensity distribution of each of the light beams.
According to a second aspect of the present invention, in the first aspect of the invention, the scanning optical device is characterized in that a difference of intensities of illumination distributions of the light beams in both end parts of an effective scanning region on the surface to be scanned is within 4%.
According to a third aspect of the present invention, in the first or second aspects of the invention, the scanning optical device is characterized in that the first optical system includes a third optical system and a fourth optical system, and an optical axis of the third optical system which guides a light beam emitted from one of the at least two light emitting points to the deflection means and an optical axis of the fourth optical system which guides a light beam emitted from the other light emitting point to the deflection means are incident to the deflection surface of the deflection means while forming a predetermined opening angle within the main scanning section.
According to a fourth aspect of the present invention, in the first or second aspects of the invention, the scanning optical device is characterized in that the at least two light emitting points are included in a monolithic multi-semiconductor laser.
According to a fifth aspect of the present invention, in the first or second aspects of the invention, the scanning optical device is characterized in that the at least two light beams emitted from the at least two light emitting points are synthesized by a beam synthesizing system in the first optical system and guided onto the deflection surface of the deflection means.
According to a sixth aspect of the present invention, in any one of the first to fifth aspects of the invention, the scanning optical device is characterized in that when an incident optical axis for beam incident to the deflection surface of the deflection means of the first optical system and an optical axis of the second optical system are projected onto the main scanning section, the two optical axes substantially coincide with each other.
According to a seventh aspect of the present invention, in any one of the first to sixth aspects of the invention, the scanning optical device is characterized in that the at least two light beams emitted from the at least two light emitting points are incident from an oblique direction with respect to the deflection surface of the deflection means within a sub scanning section.
According to an eighth aspect of the present invention, in any one of the first to seventh aspects of the invention, the scanning optical device is characterized in that the at least two light emitting points are apart from each other in at least a main scanning direction.
According to a ninth aspect of the present invention, in the eighth aspect of the invention, the scanning optical device is characterized in that an angle formed between principal rays forming the largest angle therebetween among principal rays of the at least two light beams which are emitted from the at least two light emitting points and incident to the deflection surface of the deflection means is 0 degree to 6 degrees.
According to a tenth aspect of the present invention, in any one of the first to seventh aspects of the invention, the scanning optical device is characterized in that the at least two light emitting points are apart from each other only in a sub scanning direction.
According to an eleventh aspect of the present invention, in any one of the first to tenth aspects of the invention, the scanning optical device is characterized in that: the at least two light emitting points are included in a semiconductor laser; and the deflection surface of the deflection means is moved within a range in which the amount of light beam is xc2xd or more of a maximum value of a light amount distribution of each of the at least two light beams incident to the deflection surface of the deflection means in the main scanning section.
According to a twelfth aspect of the present invention, in any one of the first to eleventh aspects of the invention, the scanning optical device is characterized by further including at least three light emitting points.
According to a thirteenth aspect of the present invention, there is provided an image forming apparatus characterized by including:
the scanning optical device according to any one of the first to twelfth aspects;
a photosensitive member located on the surface to be scanned;
a developing unit for developing as a toner image an electrostatic latent image formed on photosensitive member using the light beams scanned by the scanning optical device;
a transferring unit for transferring the developed toner image to a material to be transferred; and
a fixing device for fixing the transferred toner image onto the material to be transferred.
According to a fourteenth aspect of the present invention, there is provided an image forming apparatus characterized by including:
a scanning optical device according to any one of the first to twelfth aspects; and
a printer controller for converting code data inputted from an external device into an image signal and inputting the image signal to the scanning optical device.